Scott Akerman explains the fine art of sail trim. Originally published in Blue Water Sailing.
When sailing around with friends or clients, I’m often critical of how other vessels trim their sails. Anyone who has been aboard with me knows that the sailing instructor in me instantly diagnoses the problem and goes through the machinations to fix it.
Mainsail twist is measured by getting under the boom and looking up the leech to see where the top batten—or 20%—of the sail is pointing. If it’s not parallel with the boom and is headed away from it, there is a certain amount of twist. If you ever sail a boat with no vang on a beam or broad reach, you’ll see that the top part of the sail is often up against the spreaders, while the bottom is closer in toward the hull. Not efficient!
Also inefficient is having the main overtrimmed so that the top batten is pointing back into the boat. This will stall out the top of the sail and slow you down. The goal is to have “attached flow” so the wind is going across the sail and reattaching at the trailing edge, providing you with maximum lift. If you are over-trimmed, you’ll notice that the telltales on the trailing edge of your sail (if you have any) are stalled and not flying straight back. I have a rule: “When in doubt, let it out.” This means that the main should be trimmed on the verge of a luff so that you’re getting the most lift without generating too much weather helm.
The best ways to control twist in the main are with the traveller, the vang and sometimes the preventer. When close-hauled or close-reaching, the traveller is almost directly below the boom and has the best leverage to control the twist. If you are in gusty conditions, or if there is a lot of shear or gauge (where there is more wind aloft than at sea level), you might want to twist off the top to limit the gusts making you heel too much and having to steer constantly. Every time you turn the wheel, you’re creating drag on your rudder and slowing yourself down.
If you are reaching, the vang has better leverage to hold the boom down and limit the twist. If you are running and have a preventer on—depending on where it’s fitted to the deck—it, too, can hold the boom down and control twist.
Your head sail’s twist controls are your fairleads, sheets and barberhauler. Once again, in order to evaluate twist, have a look at the telltales along the headsail from top to bottom. If you see the top telltale break earlier than the bottom, you’ve got some amount of twist. The general goal is to have all telltales break at the same time so you get the most performance out of your sail plan.
In order to modify twist, you’ll need to move the fairleads forward or aft depending on what you’re trying to do. If you cannot move the fairleads because they are stuck, you have not moved them enough!
As a rule, while broad reaching I move the leads as far forward as I can to eliminate twist and catch as much breeze as possible from the top of the sail. As you change your point of sail to a beam reach or close reach, you’ll have to experiment to see where the sweet spot is.
As the jib car is loaded, either move the lazy sheet car and tack or gybe over to evaluate. Next, furl in the sail and unfurl once moved. Use a short sheet to unload the sheet, or maybe you have a fairlead with a ratio purchase to make it easier to move while loaded.
Once again, if you see gusty conditions you’ll want to have the top twisted off to move the center of effort lower and minimize heeling and steering. In that case, let the middle of the sail do the work and get that set of telltales to flow straight back to maximize lift.
If you tighten the sheet too much, you may “close the slot.” The slot is the area between the main and the headsail. It’s important to keep that space open as it’s the funneling of air over your sailplan that’s giving you lift. You’ll find yourself tweaking the sheets and leads to figure out what will give you the best performance from your sailplan.
I know this all sounds like a lot of work, but you’ll see a knot gain to your boat speed here and there, and one knot over 1000 nautical miles translates to a lot of hours.
Scott Akerman has been teaching sailing since 1979. He is a US Sailing instructor and has 100,000+ miles as a yacht captain.
